[FCP++] Set FCP++ to stable

FCP++ has already been 50% enabled in stable for a while. As we don't
see performance regression, we should push FCP++ to 100%. According to
the best practice, we should set FCP++ to stable on ToT before enabling
the finch trial for 100%.

Bug: 966216

Change-Id: I419296a7fd21c4f697e42e3fadba8b00b5776f53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626223
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662608}

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -577,6 +577,7 @@
     // metric. See also: http://bit.ly/fcp_plus_plus
     {
       name: "FirstContentfulPaintPlusPlus",
+      status: "stable",
     },
     {
       name: "FocuslessSpatialNavigation",